Why can't I move my folder from iCloud Drive to my hard drive?

Some things to try.

In the absence of more information and assuming you attempted to copy to different local drives I would initially suspect the problem to be network/iCloud related. Especially since you recently ran out of space.

However, worth covering all basis.

1. Make sure you are not moving around files still in sync process, and you have enough space in iCloud for all the uploads you enabled.
2. Make absolutely sure local target is not located somewhere that sync back to iCloud
3. Make sure you copy and not move the files (default is move). You can delete later.
4. Try do download through Safari (hopefully you have a flat file structure as one cannot download folders through the browser).
5. There are various reports and discussions of High Sierra especially on APFS drives reporting free disc space differently depending on which tools you use. Compare the free space reported in Disc Utility with what is reported in System Report -> Storage, and Terminal "df -h" command.
6. Rebuild Spotlight Index. How to rebuild the Spotlight index on your Mac - Apple Support
7. Empty trash
8. If you have multiple backup discs, make all backups are up to date. Also check for snapshots using tmutil, flatten if needed. https://ppolyzos.com/2017/10/20/how-to-manually-delete-local-snapshots-from-time -machines-backups/
9. Check S.M.A.R.T health status of disk.

Ok, given the free space checks all indicate enough space. The "100%" items you see in df is normal and not real disk space. You wanted to confirm /dev/disk0s2 and /dev/disk1s2 and they look good, even if they report different free space, there is plenty.

yes, I meant icloud.com to download through Safari.
This directories you see in iCloud, being the root, do seem out of place. Maybe some accidental drop of files from some cache or directory? I recall seen similar directory names in a .Spotlight cache hierarch. Seen similar directory names under the InstallShield directories in MS Windows.
Other than the app specific directories like Automator, Keynote, etc, the only content in root I see on my root iCloud drive is what I put there.

I think safe to move on and focus on that sync and what is going on with iCloud.

To explain why I suggest copy rather than move. Apologies if you know this already, I don't your skill level. Usually a move action between different filesystems is actually simply a copy + delete action performed by the tool you use. Some problems may occur with the "delete" part (space overheads, permissions, etc). That is why first thing with weird move issues I do is seperate the two actions into a manual actions of copy and delete.

That directory you are trying to copy looks like a lot of files for only a few MB of data. Is that normal for XCode?

I don't know the inner working of iCloud well enough to comment much there but if this was between two local disks I would immediately suspect the two scenarios I personally ran into in the past.

1. Circular symbolic links may cause a loop resulting in a endless loop of file system navigation.
2. Trying to copy files from a Time Machine backup with tools other than TM. As TM use a special kind of hard link on HFS+ partitions regular tools will result in significant duplication.

If you have good backups of your data I would at this point maybe turn off as much iCloud as I can, let things settle, clean up iCloud first and then one-by-on enable iCloud sync again making sure enough space.

Where is that folder located on iCloud drive, and where in your mac are you trying to copy it to?

Do you have, in iCloud options, have your Documents & Desktop set to be in iCloud?

Depending on the allocated space and in-sync items that 10% could be gone before the email reach your inbox.
If not, noted 🙂